Mine wastewater treatment using Phalaris arundinacea plant material hydrolyzate as substrate for sulfate-reducing bioreactor

Effect of different carbon sources on sulfate reduction and microbial community structure in bioelectrochemical systems

Microbial electrolysis cells (MECs) have rapidly developed into a promising technology to treat sulfate-rich wastewater that lacks electron donors. Hence, a better understanding of the effect on the microbial community structure caused by different sources in bioelectrochemical systems is required. This study sought to investigate the effect of different carbon sources (NaHCO₃, ethanol, and acetate were employed as sole carbon source respectively) on the performance of sulfate-reducing biocathodes. The sulfate reduction efficiency enhanced by the bioelectrochemical systems was 8.09 - 11.57% higher than that of open-circuit reference experiments. Furthermore, the optimum carbon source was ethanol with a maximum sulfate reduction rate of 170 mg L^-1 d^-1 in the bioelectrochemical systems. The different carbon sources induced significant differences in sulfate reduction efficiency as demonstrated by the application of a micro-electrical field. Microbial community structure and network analysis revealed that all three kinds of carbon source systems enriched large proportions of sulfate-reducing bacteria and electroactive bacteria but were significantly distinct in composition. The dominant sulfate-reducing bacteria that use NaHCO₃ and acetate as carbon sources were Desulfobacter and Desulfobulbus, whereas those that use ethanol as carbon source were Desulfomicrobium and Desulfovibrio. Our results suggest that ethanol is a more suitable carbon source for sulfate reduction in bioelectrochemical systems.

Valorization of wastewater: A paradigm shift towards circular bioeconomy and sustainability

Limitation in the availability of natural resources like water is the main drive for focussing on resource recovery from wastewater. Rapid urbanization with increased consumption of natural resources has severely affected its management and security. The application of biotechnological processes offers a feasible approach to concentrating and transforming wastewater for resource recovery and a step towards a circular economy. Wastewater generally contains high organic materials, nutrients, metals and chemicals, which have economic value. Hence, its management can be a valuable resource through the implementation of a paradigm transformation for value-added product recovery. This review focuses on the circular economy of "close loop" process by wastewater reuse and energy recovery identifying the emerging technologies for recovering resources across the wastewater treatment phase. Conventional wastewater treatment technologies have been discussed along with the advanced treatment technologies such as algal treatment, anammox technology, microbial fuel cells (MFC). Apart from recovering energy in the form of biogas and biohydrogen, second and third-generation biofuels as well as biohythane and electricity generation have been deliberated. Other options for resource recovery are single-cell protein (SCP), biopolymers as well as recovery of metals and nutrients. The paper also highlights the applications of treated wastewater in agriculture, aquaponics, fisheries and algal cultivation. The concept of Partitions-release-recover (PRR) has been discussed for a better understanding of the filtration treatment coupled with anaerobic digestion. The review provides a critical evaluation on the importance of adopting a circular economy and their role in achieving sustainable development goals (SDGs). Thus, it is imperative that such initiatives towards resource recovery from wastewater through integration of concepts can aid in providing wastewater treatment system with resource efficiency.

Comparison of different small molecular weight alcohols for sustaining sulfidogenic bioreactors maintained at moderately low pH

Sulfate-reducing bacteria (SRB) catalyse the dissimilatory reduction of sulfate to hydrogen sulfide using a wide range of small molecular weight organic compounds, and hydrogen, as electron donors. Here we report the effects of different combinations of small molecular weight alcohols on the performance and bacterial composition of a moderately low pH sulfidogenic bioreactor (pH 4.0–5.5) operated at 35°C in continuous flow mode. Ethanol alone and methanol or ethanol used in combination with glycerol were evaluated based on their equivalent amounts of carbon. Although evidenced that methanol was utilised as electron donor to fuel sulfidogenesis at pH 5.5, rates of sulfate reduction/sulfide production were negatively impacted when this alcohol was first introduced to the system, though these rates increased in subsequent phases as a result of adaptation of the microbial community. Further increased dosage of methanol again caused rates of sulfidogenesis to decrease. Methanol addition resulted in perturbations of the bioreactor microbial community, and species not previously detected were present in relatively large abundance, including the sulfate-reducer Desulfovibrio desulfuricans. Ethanol utilization was evidenced by the increase in rates of sulfidogenesis as the dosage of ethanol increased, with rates being highest when the bioreactor was fed with ethanol alone. Concentrations of acetate in the effluent liquor also increased (up to 8 mM) as a result of incomplete oxidation of ethanol. This alcohol continued to be used as the electron donor for sulfate reduction when the bioreactor pH was decreased incrementally (to pH 4.0), but rates of sulfidogenesis decreased. The relative abundance of Dv. desulfuricans diminished as the bioreactor pH was lowered, while that of the acidophilic Firmicute Desulfosporosinus acididurans increased. This study has shown that all three alcohols can be used to fuel microbial sulfidogenesis in moderately acidic liquors, though the cost-effectiveness, availability and toxicity to the microbial community will dictate the choice of substrate.

Advances in heavy metal removal by sulfate-reducing bacteria

Industrial development has led to generation of large volumes of wastewater containing heavy metals, which need to be removed before the wastewater is released into the environment. Chemical and electrochemical methods are traditionally applied to treat this type of wastewater. These conventional methods have several shortcomings, such as secondary pollution and cost. Bioprocesses are gradually gaining popularity because of their high selectivities, low costs, and reduced environmental pollution. Removal of heavy metals by sulfate-reducing bacteria (SRB) is an economical and effective alternative to conventional methods. The limitations of and advances in SRB activity have not been comprehensively reviewed. In this paper, recent advances from laboratory studies in heavy metal removal by SRB were reported. Firstly, the mechanism of heavy metal removal by SRB is introduced. Then, the factors affecting microbial activity and metal removal efficiency are elucidated and discussed in detail. In addition, recent advances in selection of an electron donor, enhancement of SRB activity, and improvement of SRB tolerance to heavy metals are reviewed. Furthermore, key points for future studies of the SRB process are proposed.

Engineering and kinetic aspects of bacterial uranium reduction for the remediation of uranium contaminated environments

Biological reduction of soluble uranium from U(VI) to insoluble U(IV) coupled to the oxidation of an electron donor (hydrogen or organic compounds) is a potentially cost-efficient way to reduce the U concentrations in contaminated waters to below regulatory limits. A variety of microorganisms originating from both U contaminated and non-contaminated environments have demonstrated U(VI) reduction capacity under anaerobic conditions. Bioreduction of U(VI) is considered especially promising for in situ remediation, where the activity of indigenous microorganisms is stimulated by supplying a suitable electron donor to the subsurface to contain U contamination to a specific location in a sparingly soluble form. Less studied microbial biofilm-based bioreactors and bioelectrochemical systems have also shown potential for efficient U(VI) reduction to remove U from contaminated water streams. This review compares the advantages and challenges of U(VI)-reducing in situ remediation processes, bioreactors and bioelectrochemical systems. In addition, the current knowledge of U(VI) bioreduction mechanisms and factors affecting U(VI) reduction kinetics (e.g. pH, temperature, and the chemical composition of the contaminated water) are discussed, as both of these aspects are important in designing efficient remediation processes.

Metal biorecovery and bioremediation: Whether or not thermophilic are better than mesophilic microorganisms

Metal mobilization and immobilization catalyzed by microbial action are key processes in environmental biotechnology. Metal mobilization from ores, mining wastes, or solid residues can be used for recovering metals and/or remediating polluted environments; furthermore, immobilization reduces the migration of metals; cleans up effluents plus ground- and surface water; and, moreover, can help to concentrate and recover metals. Usually these processes provide certain advantages over traditional technologies such as more efficient economical and environmentally sustainable results. Since elevated temperatures typically increase chemical kinetics, it could be expected that bioprocesses should also be enhanced by replacing mesophiles with thermophiles or hyperthermophiles. Nevertheless, other issues like process stability, flexibility, and thermophile-versus-mesophile resistance to acidity and/or metal toxicity should be carefully considered. This review critically analyzes and compares thermophilic and mesophilic microbial performances in recent and selected representative examples of metal bioremediation and biorecovery.

Lignocellulosic biowastes as carrier material and slow release electron donor for sulphidogenesis of wastewater in an inverse fluidized bed bioreactor

Industrial wastewaters containing high concentrations of sulphate, such as those generated by mining, metallurgical and mineral processing industries, require electron donor for biological sulfidogenesis. In this study, five types of lignocellulosic biowastes were characterized as potential low-cost slow release electron donors for application in a continuously operated sulphidogenic inverse fluidized bed bioreactor (IFBB). Among them, natural scourer and cork were selected due to their high composition of volatile solids (VS), viz. 89.1 and 96.3%, respectively. Experiments were performed in batch (47 days) and in an IFBB (49 days) using synthetic sulphate-rich wastewater. In batch, the scourer gave higher sulphate reduction rates (67.7 mg SO₄^2- L^-1 day^-1) in comparison to cork (12.1 mg SO₄^2- L^-1 day^-1), achieving >82% sulphate reduction efficiencies. In the IFBB packed with the natural scourer, the average sulphate reduction efficiency was 24 (±17)%, while the volumetric sulphate reduction rate was 167 (±117) mg SO₄^2- L^-1 day^-1. The long incubation time in the batch experiments (47 days) allowed higher sulphate reduction efficiencies in comparison to the short hydraulic retention time (24 h) in the IFBB. This suggests the hydrolysis-fermentation was the rate-limiting step and the electron donor supply (through hydrolysis of the lignocellulosic biowaste) was limiting the sulphate reduction. Lignocellulose as carrier material and slow release electron donor for sulphidogenesis.

Carbohydrate based polymeric materials as slow release electron donors for sulphate removal from wastewater

Many industrial sulphate rich wastewaters are deficient in electron donors to achieve complete sulphate removal. Therefore, pure and expensive chemicals are supplied externally. In this study, carbohydrate based polymers (CBP) as potato (2 and 5 mm³), filter paper (2 and 5 mm²) and crab shell (2 and 4 mm Ø) were tested as slow release electron donors (SRED) for biological sulphate reduction at 30 °C and initial pH of 7.0. Using the CBP as SRED, sulphate reduction was carried out at different rates: filter paper 0.065-0.050 > potato 0.022-0.034 > crab shell 0.006-0.009 mg SO₄^2-.mg VSS^-1d^-1. These were also affected by the hydrolysis-fermentation rates: potato 0.087-0.070 > filter paper 0.039-0.047 > crab shell 0.011-0.028 mg COD_S.mg VSS^-1d^-1, respectively. Additionally, the sulphate removal efficiencies using filter paper (cellulose, > 98%), potato (starch, > 82%) and crab shell (chitin, > 32%) were achieved only when using CBP as SRED and in the absence of other easily available electron donors. This study showed that the natural characteristics of the CBP limited the hydrolysis-fermentation step and, therefore, the sulphate reduction rates.

Improvement of the degradation of sulfate rich wastewater using sweetmeat waste (SMW) as nutrient supplement

External dosing of sweetmeat waste (SMW) dosing into exhausted upflow packed bed bioreactor (PBR) resulted in prompt reactivation of SO4(2-) removal. Different SMW concentrations in terms of chemical oxygen demand (COD)/SO4(2-) ratios (1, 2, 4 and 8) were introduced into four identical PBR where process stability was found within 3 weeks of operation. SO4(2-) removal was proportional to COD/SO4(2-) ratios up to 4 at which maximum sulfate removal (99%) was achieved at a rate of 607 mg/d. The value of COD consumption:SO4(2-)removal was much higher at ratio 4 than 8 whereas, ratio 2 was preferred over all. Net effluent acetate concentration profile and total microbial population attached to the reactor matrices were corresponding to COD/SO4(2-) ratio as 4>8>2>>1. Sulfate reducing bacteria (SRB) population was found to be inversely proportional to COD/SO4(2-) ratio in which acetate oxidizing SRB and fermentative bacteria were the dominant.

A review of biological sulfate conversions in wastewater treatment

Treatment of waters contaminated with sulfur containing compounds (S) resulting from seawater intrusion, the use of seawater (e.g. seawater flushing, cooling) and industrial processes has become a challenging issue since around two thirds of the world's population live within 150 km of the coast. In the past, research has produced a number of bioengineered systems for remediation of industrial sulfate containing sewage and sulfur contaminated groundwater utilizing sulfate reducing bacteria (SRB). The majority of these studies are specific with SRB only or focusing on the microbiology rather than the engineered application. In this review, existing sulfate based biotechnologies and new approaches for sulfate contaminated waters treatment are discussed. The sulfur cycle connects with carbon, nitrogen and phosphorus cycles, thus a new platform of sulfur based biotechnologies incorporating sulfur cycle with other cycles can be developed, for the removal of sulfate and other pollutants (e.g. carbon, nitrogen, phosphorus and metal) from wastewaters. All possible electron donors for sulfate reduction are summarized for further understanding of the S related biotechnologies including rates and benefits/drawbacks of each electron donor. A review of known SRB and their environmental preferences with regard to bioreactor operational parameters (e.g. pH, temperature, salinity etc.) shed light on the optimization of sulfur conversion-based biotechnologies. This review not only summarizes information from the current sulfur conversion-based biotechnologies for further optimization and understanding, but also offers new directions for sulfur related biotechnology development.

The use of low-cost adsorbents for wastewater purification in mining industries

Recently, great attention has been paid to the environmental problems in mining industry. At present there are different ways of mineral processing, as well as various methods of wastewater treatment, most of them are expensive. Work is ongoing to find low-cost treatments. In this article, low-cost adsorbents, potentially useful for wastewater treatment on mining and metallurgical plants, are reviewed; their characteristics, advantages, and disadvantages of their application are compared. Also adsorption of different metals and radioactive compounds from acidic environment similar to composition of mining and metallurgical wastewaters is considered.

Comparative effectiveness of mixed organic substrates to mushroom compost for treatment of mine drainage in passive bioreactors

Bioreactors are one possible best sustainable technology to address the mine-impacted water problems. Several prospective substrates (mushroom compost, cow manure, sawdust, wood chips, and cut rice straw) were characterized for their ability to serve as a source of food and energy for sulfate-reducing bacteria. Twenty bench-scale batch bioreactors were then designed and set up to investigate relative effectiveness of various mixtures of substrates to that of mushroom compost, the most commonly used substrate in field bioreactors, for treating mine drainage with acidic (pH 3) and moderate pH (pH 6). Overall, reactive mixtures showed satisfactory performances in generating alkalinity, reducing sulfate and removing metals (Al>Fe>Mn) (up to 100%) at both pH conditions, for all substrates. The mixture of sawdust and cow manure was found as the most effective whereas the mixture containing 40% cut rice straw gave limited efficiency, suggesting organic carbon released from this substrate is not readily available for biodegradation under anaerobic conditions. The mushroom compost-based bioreactors released significant amount of sulfate, which may raise a more concern upon the start-up of field-scale bioreactors. The correlation between the extent of sulfate reduction and dissolved organic carbon/SO(4)(2-) ratio was weak and this indicates that the type of dissolved organic carbon plays a more important role in sulfate reduction than the absolute concentration and that the ratio is not sensitive enough to properly describe the relative effectiveness of substrate mixtures.

Sulfidogenic fluidized bed treatment of real acid mine drainage water

The treatment of real acid mine drainage water (pH 2.7-4.3) containing sulfate (1.5-3.34 g/L) and various metals was studied in an ethanol-fed sulfate-reducing fluidized bed reactor at 35°C. The robustness of the process was tested by increasing stepwise sulfate, ethanol and metal loading rates and decreasing feed pH and hydraulic retention time. Highest sulfate reduction rate (4.6g/L day) was obtained with feed sulfate concentration of 2.5 g/L, COD/sulfate ratio of 0.85 and HRT of 12 h. The corresponding sulfate and COD removal efficiencies were about 90% and 80%, respectively. The alkalinity produced in sulfidogenic ethanol oxidation neutralized the acidic mine water. Highest metal precipitation efficiencies were observed at HRT of 24 h, the percent metal removal being over 99.9% for Al (initial concentration 55 mg/L), Co (9.0 mg/L), Cu (49 mg/L), Fe (435 mg/L), Ni (3.8 mg/L), Pb (7.5 mg/L) and Zn (6.6 mg/L), and 94% for Mn (7.21 mg/L).

Silage supports sulfate reduction in the treatment of metals- and sulfate-containing waste waters

Silage was used as source of carbon and electrons for enrichment of silage-degrading and sulfate reducing bacteria (SRB) from boreal, acidic, metals-containing peat-bog samples and to support their use in batch and semi-batch systems in treatment of synthetic waste water. Sulfidogenic silage utilization resulted in a rapid decrease in lactate concentrations; concentrations of acetate, butyrate and propionate increased concomitantly. Synthetic waste water consisting of Mn, Mg and Fe (II) ions inhibited sulfate reduction at concentrations of 6 g/l, 8 g/l and 1 g/l respectively. During treatment, Mn and Mg ions remained in solution while Fe ions partially precipitated. Up to 87 mg sulfate was reduced per gram of silage. Sulfate reduction rates of 34, 22 and 6 mg/l/day were obtained at temperatures of 30, 20 and 9 °C respectively. In semi-batch reactors operated at low pH, the iron precipitation capacity was controlled by sulfate reduction rates and by partial loss of hydrogen sulfide to the gas phase. Passive reactor systems should, therefore, be operated at neutral pH. Metals tolerant, silage-fermenting (predominantly species belonging to genus Clostridium) and sulfate reducing bacteria (including a species similar to the psychrotolerant Desulfovibrio arcticus) were obtained from the peat bog samples. This work demonstrates that silage supports sulfate reduction and can be used as a low cost carbon and electron source for SRB in treatment of metals-containing waste water.